Ultrasonic imaging technology has become a vital tool for examining the internal structure of living organisms. For the diagnosis of various medical conditions, ultrasonic imaging is often useful to examine soft tissues within the body to show the structural detail of internal tissues and fluid flow.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties of the echoes (e.g., amplitude and phase) from tissues within the body. Typically, the ultrasound beam is focused at various steps within the body to improve resolution or image quality. Echoes are received by the transducer and processed to generate an image of the object. The resulting image is usually referred to as a B-scan image.
Measuring and imaging blood (and other bodily fluid) flow within a living subject is typically done using the Doppler principle, wherein a transmitted burst of ultrasound at a specific frequency is reflected from moving blood cells, thereby changing the frequency of the reflected ultrasound in accordance with the velocity in the direction of the flow. The frequency shift (Doppler shift) of reflected signals with respect to the transmitted signals is proportional to the velocity of the fluid flow. This frequency may be detected and displayed on a video display to provide graphic images of moving tissue structure and fluid flow within a living patient.
Present ultrasound techniques include frequency-shift color Doppler and power color Doppler imaging of tissue motion, as well as the cross-correlation ultrasound estimation of displacements and mean velocities for color mapping tissue motion (referred to as CVI and developed by Philips Corp.). Another known technique is the single or multi-compression cross correlation ultrasound elastography method using radio frequency (RF) signals.
Present known methods of ultrasound tissue imaging using color Doppler techniques and cross-correlation exhibit relatively poor signal-to-noise ratios because small displacement, low-velocity tissue motions are relatively close to the noise floor. The multi-compression, cross-correlation approach requires radio frequency data and is very computationally intensive, presently requiring non-real time, off-line processing.
These present ultrasound methods are directed toward estimating the direction and velocity of fluid flow within a body, and are therefore not optimized for detecting and measuring certain mechanical properties of the tissue being scanned. The mechanical properties not adequately detected by present ultrasound methods include elasticity, inertia, resonance, and damping characteristics of tissue media. An accurate evaluation of these mechanical properties of soft tissue within a subject can contribute greatly to understanding the condition and identity of the tissues and structures being scanned.
However, because these mechanical properties exhibit only a small measure of vibrational motion relative to typical fluid flow within a body, adequate detection requires devices which are more sensitive than existing systems. Although certain elastographic imaging systems exist, they involve complex circuitry and do not provide real-time analysis.
It is therefore an intended advantage of the present invention to provide a device which performs Doppler sonography of small displacement, low-velocity tissue motion with increased signal-to-noise ratio in the Doppler signal path.
It is a further intended advantage of the present invention to provide a an ultrasound device which performs real-time analysis of certain mechanical properties of tissue.
It is yet a further intended advantage to provide a method of performing elastographic imaging using existing ultrasound systems and a relatively simple and easy to use attachment device.